Beer is commonly packaged in openable sealed cans which are under an internal pressure of the carbon dioxide in the beer. Leaks sometimes develop in such cans causing loss of carbon dioxide, resulting in "flat" beer in the cans. However, it is not readily apparent to the consumer whether the beer is fresh and sparkling or stale and flat without opening of the can. Can leaks do not usually provide an externally visible trace of the same. A similar problem may arise with any carbonated drink packaged in a can and the present invention also is applicable thereto.